1. Field of the Invention:
The present invention relates to underground storage tank systems and to an improved capacitance level probe used to detect changes in liquid level within an underground storage tank.
2. Description of the Prior Art:
Underground storage tank systems are commonly employed for the storage of petroleum, hazardous chemicals and other products. For instance, most gasoline service stations, truck stops and similar facilities employ underground fuel storage tanks which are refilled by means of a riser pipe which extends upwardly from the buried tank into a relatively shallow manhole in the concrete apron of the location. The sides of the manhole are spaced a sufficient radial distance from the riser pipe to provide convenient access for coupling and uncoupling the supply hose of a supply truck to the top of the riser pipe. A fill tube extends within the riser pipe into the interior of the tank and is spaced concentrically within the riser pipe to thereby define an annular space between the fill tube and the riser pipe. The bottom of the manhole can be open or closed, e.g., cemented or filled with dirt or a mixture of pea gravel.
It is often advantageous to install a product monitoring device, such as a level monitoring device, within the interior of the underground storage tank, e.g., to report product levels for inventory control purposes, or in special operating modes to detect tank leakage.
The present invention has as its object to provide an improved capacitance level probe of the type which can be installed within an underground storage tank to monitor changes in liquid level.
The present invention also has as its object to provide a capacitance level probe which can be installed within the annular space between the riser pipe and fill tube with a minimum disruption to the existing storage tank installation, including the existing concrete apron which covers the buried storage tank.
Another object of the invention is to provide an improved capacitance level probe which accurately monitors changes in liquid level within an underground fuel storage tank, even during refueling operations when levels are changing rapidly.
Another object of the invention is to provide such a capacitance level probe which can be mounted at least partially within the annular space between the riser pipe and fill tube and which includes a pressure drop equalization feature to accommodate changes in pressure caused by changing velocity of vapors travelling through the annular space in a coaxial vapor recovery refueling arrangement.
Additional objects, features and advantages will be apparent in the written description which follows.